Advantage of helical gears

During internal viva, we were asked what is the particular advantage of using helical gears over spur gears. The guy next to me answered that it has higher load capacity given similar dimensions, similar material etc. Professor accepted his answer. I was kinda like:surprised

When I was asked, I said they have similar load capacities, but because of gradual loading, the wear is less, operation is quieter & a bit lesser dynamic load. So he reasoned because a helical gear has more number of teeth in contact, it has higher load capacity.

But when a spur gear transmits power, one full tooth transfers power, while in case of helical gears, part of 3-4 teeth will be engaged & will transmit power, & still would add up to only one complete tooth. So the professor was like ":zzz:you are wrong!!".

You are correct. They have more teeth in contact, but the entire face of the tooth is not in contact. They roll through the face. Now they do act quieter and do have advantages.

Example. We use a program which automatically calculates AGMA allowable power and so on and so forth. We have a standard spur gear set with a 25° pressure angle. If I keep EVERYTHING the same and change from spur to a 10° helical gear, the allowable limiting horsepower for the gear set drops about 17%. At 15° helical angle, the drop is almost 25%.